Atomic physicists have traditionally studied the properties and interactions of small numbers of atoms and molecules. Condensed-matter physicists, on the other hand, are interested in the behaviour of larger numbers of such particles. In recent years, however, the interests of these two groups of physicists have started to converge. Clusters, for example, bridge the gap between small numbers of atoms and the solid state, as do Bose-Einstein condensates - a new form of matter in which millions of atoms collapse into the same quantum state. Many of the laser cooling and trapping techniques that have helped to make Bose-Einstein condensation possible are also being used to explore the interface between atomic and bulk properties in another regime - in the new state of matter found in "optical lattices".
This new matter consists of ultracold atoms sitting in a crystal-like array in an optical potential where the intensity or polarization of the light varies periodically. The atoms are not held together by the customary action of chemical bonds - as in a crystal - but by the light itself. This novel matter was first created in 1992 by physicists in France and the US who used a web of interfering laser beams to cool a collection of atoms to near absolute zero. They found that the atoms became suspended in well defined positions in the interfering beams. Seeing the obvious analogy with a conventional crystal, they dubbed the optical potential that traps the atoms an "optical lattice".
In the October issue of Physics World magazine (information), Steven Rolston from the National Institute of Standards and Technology, Gaithersburg, US writes about the latest research in optical lattices.